1. Field of the Disclosure
The disclosure is generally related to the field of water and wastewater treatment. More specifically, the disclosure relates to novel apparatus and methods of using same which address problems of removal of solids and chemically-induced flocs from residential, municipal, and/or industrial wastewater.
2. Description of Related Art
A conventional method for reducing solids from residential, municipal, and industrial wastewater is by using a cloth disk filter. Cloth disk filters are gaining in popularity in municipal tertiary and industrial wastewater filtration because of their low capital cost, acceptable solids removal efficiency, low filtration headloss, low reject rate (little or no chemical treatment) and small footprint.
A prior art cloth disk filter is illustrated schematically in FIG. 1, with parts cut away so that some of the key components may be better understood. The prior art cloth disk filter embodiment 100 comprises a tank or vessel 2, which may be concrete, metal, fiberglass-reinforced plastic (FRP) or other material. A source of screened, raw wastewater flows into the unit from an influent trough 4, and clean effluent leaves the unit through an effluent trough 6. Influent traverses an influent weir 8, encounters a scum or hydraulic optimization baffle 10, then approaches one or more filter cassettes 12.
As illustrated in FIGS. 2 and 3, a filter cassette 12 comprises a rigid support 14 (FIG. 2) and a filter cloth 16 (FIG. 3). Rigid support 14 includes, in this embodiment, an upper rigid grid 18 having a plurality of ribs 19, and two lower rigid grids 20 and 22, each having respective sets of ribs 21 and 23. Ribs 19, 21, and 23 form a plurality of slots 27 in upper and lower rigid grids, allowing liquid to flow through and up to an outlet 26, and finally through an effluent conduit 28. A space 24 is provided in rigid support 14 to allow the rigid support to straddle a central backwash conduit 52 as further explained herein. A lifting hook 30 is provided for lifting the cassettes out of the vessel and installing them into the vessel. Filter cloth 16 (FIG. 3) includes two legs 32 and 34, each having a front and back side (only the front is illustrated), the front and back sides fitting snuggly over lower rigid grids 20, 22 of rigid support 14. Front and back sides are held together by peripheral zippers 36, 37. Filter cloth 16 also typically includes strengthening materials 38, 40, which may be hook and loop fasteners, and a closure 42 which fits around outlet 26 of rigid support 14.
Referring again to FIG. 1, filter cassettes 12 are each held in vessel 2 by respective upper support hanger 44 and a lower support hanger 46. A longitudinal support beam 48 supports a vertical support column 50 (one column 50 for each cassette 12). A central backwash conduit 52 is connected to a sprocket/chain/motor assembly 54, 56, and 58, which also include a smaller drive sprocket 60. Vertical support columns 50 support central backwash conduit 52 and allow conduit 52, and components attached to it, to be rotated using the sprocket/chain/motor assembly, while the filter cassettes 12 remain stationary. Each cassette 12 has at least one dedicated backwash vacuum shoe assembly 62 positioned on one side of filter cloth 16 fluidly attached to central backwash conduit 52. In some embodiments, another dedicated backwash vacuum shoe assembly (not illustrated in FIG. 1) is positioned on the reverse side of each cassette, and positioned 180 degrees out of phase, so that when one shoe assembly is pointing in a first direction, its complementary shoe assembly is pointing in the opposite direction. (In some other prior art embodiments the filter cassettes 12 rotate during backwashing, while the backwash shoes remain stationary.) Various valves V1, V2, V3, V4, and V5 illustrated in FIG. 1 may be used for various purposes recognized by those skilled in this art, such as throttling or completely blocking flow, diverting flow, taking components out of service, and the like.
FIGS. 4 and 4A illustrate further details of one embodiment of a backwash vacuum shoe assembly 62, including a vacuum shoe 64, supports 66, 68 connecting vacuum shoe 64 to a rigid vacuum conduit 71, and a flexible vacuum hose 70 fluidly connecting rigid vacuum conduit 71 to vacuum shoe 64 through a connector 72. Fluid flows from vacuum shoe 64, through connector 72, flexible vacuum hose 70, and rigid vacuum conduit 71 during a backwashing operation is depicted by the curved arrows inside those conduits. The entire assembly 62 rotates, as shown by arrow R, by virtue of being connected to central backwash conduit 52. FIG. 4A illustrates the underside of vacuum shoe 64, illustrating that vacuum shoe 64 comprises first and second shoe components 65, 67, which define a slot 69 through which flows fluid and debris (such as floc accumulated on cloth filter 16) during backwashing.
Returning to FIG. 1, cloth disk filter 100 has a sludge settling area 80 at the bottom of vessel 2, as well as one or more sludge removal conduits 82 connecting through various valves and conduits 88 to backwash pumps 84, driven by motors 87. Pumps 84 and motors 87 are illustrated as outside of vessel 2, on a separate support 86, but in other embodiments could be inside vessel 2.
It should be understood that the cloth filter disk described in FIGS. 1-4 has many variations and configurations. In some commercial embodiments, the cassettes rotate and the backwash shoes are held stationary. The vessel may take many shapes, including round and rectangular. In some embodiments, a single effluent trough may be fed by dual banks of cloth disk filters on either side of the effluent trough. Some prior patents discussing cloth disk filters include U.S. Pat. Nos. 3,640,395; 4,090,965; 4,639,315; 5,635,066; 5,356,632; 5,409,618; 5,876,612; 6,267,879; 6,294,098; 7,300,585; and 7,537,689.
One of the main disadvantages of the cloth disk filter is the nature of the cloth disk filtration-surface filtration. The cloth medium is use to support the captured solids which deposit onto the cloth surface mainly by a straining filtering process. Once a cake layer forms on the cloth medium surface, cake filtration becomes the predominant filtering mechanism. Under the condition of disk filter filtration with chemical treatment, the cake resistance increases quickly with time because of the decreasing porosity and increasing thickness of solids deposit due to large quantity of chemical floc. This is unlike other filtration processes, such as granular media depth filtration. In depth filtration, the suspended particles can enter into the porous medium (grains) and move to the grain surface for attachment or retrain into the fluid. This process is repeated through the entire filter bed. Therefore the headloss building up in depth filtration is slower than that in surface filtration.
The frequency of employing filtration with chemical treatment is increasing and it has achieved an important role in wastewater treatment. Particularly, chemical treatment is used in phosphorous removal, algae removal, metal hydroxides removal, and high solids removal applications. In those applications, usually the cloth filter disk shows very short filter run and frequent backwash. A reject rate of 20 percent is not uncommon.
As noted in reference to the discussion of FIGS. 1-4, one commonly used cleaning mechanism is a rotary backwash arm. When filtration headloss reaches a filtration termination set point, a backwash flow control valve opens. Then a backwash pump starts and the backwash arm rotates around a central backwash conduit, described in FIG. 1. The filtered water that passed through the filter cloth is pulled in reverse direction by the backwash pump through the filter cloth. The backwash pump exerts vacuum action by pulling water from the inside of the filter cassette backwards through the cloth and into the backwash shoe, thereby removing solids captured on the filter cloth in the previous filter run. The backwash wastewater flows into the backwash arm to the central backwash conduit, at last to the backwash suction pump before discharge. In these cleaning mechanisms, the backwash arm sweeps and cleans a circle area of the square-shaped (or other-non-circular-shaped) filter cassette surface. In embodiments using square-shaped filter cassettes, because the rotating backwash arm cleans a circle area of the square shaped filter cloth surface, a significant (approximately 22.5%) of the square filter cloth area remains fouled and unavailable for filtration. The solids and sludge are built up there in a long run. It is not only a waste of filtration area but also has negative affect on filter effluent quality and aesthetics of the process. Another option presently practiced is to use a more expensive and difficult to fabricate circular disk filter cassette. In this practice, the cleaning shoe remains fixed and the circular disk filter cassette is rotated for circular disk cleaning.
U.S. Pat. Nos. 7,270,750 and 7,485,231 describe wastewater treatment processes for biologically removing wastes incorporating a “sludge blanket” in activated sludge clarification and aeration ponds. While the patents do describe use of sludge blanket filtration, there is no teaching, suggestion or other discussion of reducing the load of solids to filter media in a tertiary filtration unit, such as a cloth disk filter, using sludge blanket filtration. There would be little, if any expectation or prediction of success in incorporating a sludge blanket in a cloth disk filter, as evidenced by the lack of any discussion in the trade on the issue. Most likely this is due to experience with the problems mentioned above regarding cake resistance on cloth filter cassettes increasing quickly with time because of the decreasing porosity and increasing thickness of solids deposit due to large quantity of chemical floc. It would not be expected that addition of a sludge blanket would alleviate this problem.
There is a need in the wastewater treatment art for improved cloth filter disk apparatus and methods of employing cloth disk filters in water and wastewater treatment facilities, which feature reduced load on the cloth filter media while maintaining or increasing the reliability and consistency of the cloth disk filter to function as a tertiary filter. There is also a need for using all or substantially all of the filter surface area of cloth disk filter cassettes, and for backwash methods and apparatus which clean substantially all of the filter surfaces in an efficient manner, which may include moving the cleaning shoe rather than moving the entire filtration cassette.